1. Field of the Invention
This invention relates to plastic encapsulation of semiconductor devices,
2. Brief Description of the Prior Art
Semiconductor plastic packaging mold compound compositions generally include a mixture of ceramic particles, generally but not limited to silica or alumina, an epoxy/resin system, generally but not limited to epoxy cresol novolacs, a hardener which is generally very water absorbent, generally but not limited to phenolic resins or acid anhydrides and a catalyst, generally boron trifluoride (BF.sub.3), to determine the cure time and flow of the resin. Optional additives are rubber particles such as carboxyl terminated nitrile rubber (CTBN) or amine terminated nitrile rubber (ATBN) to increase fracture resistance or the material, increase the material viscosity and absorb moisture, flame retardants, mold release agents, coupling agents and pigment. The mold compound composition is generally from about 70 to about 90 weight percent of the ceramic particles, from about 10 to about 15 weight percent of the epoxy/resin system, from about 6 to about 8 weight percent of the hardener and from about 0.25 to about 0.50 weight percent of the catalyst. The ceramic particles have a generally smooth outer surface and are generally of somewhat spherical shape. Such compositions are generally molded over semiconductor chips and lead frames to which the chips have been bonded in standard manner to provide the completed package. The packages are later mounted to the surfaces of printed circuit boards and the like by standard soldering techniques and the like during which the packages are subjected to high temperatures.
A major problem with surface mount integrated circuits (ICs) has been moisture induced package cracking. The cracking is thought to occur when the moisture and volatiles present in the mold compound are vaporized during the solder reflow process. The vapor induces high internal stresses which are responsible for the cracking of the package. The crack develops and propagates when the strength of the mold compound material becomes less than the vapor-induced stresses. This crack may or may not be visible from the exterior of the package, but is large enough to cause substantial degradation to the reliability of the product. A cracked package will cause a device performance failure.
With the industry trend in the direction of thinner packages, numerous studies and experiments have been conducted to make the packages more robust to vapor phase reflow or infrared reflow soldering conditions. The common approach is to have simultaneous improvement on the different factors that affect package integrity. One area of focus for package improvements has been the use of highly filled mold compounds to minimize moisture absorption. A second area of focus has been to provide improved adhesion between the chip and mold compound and between the bottom die pad and mold compound, adhesion promoters and the requirement of reduced amounts of release agent making this improvement possible. A third area of improvement is being made through improvements in mold compound toughness to resist the formation and propagation of cracks. New methods have been introduced to measure the mechanical strength of the new mold compound materials in order to account for the different stresses acting internally in the package. The introduction of stress absorbers in the compound chemistry has proven to help improve its ability to resist cracking but still not guarantee to eliminate the problem. However, the problem of package cracking still exists and major efforts are ongoing to minimize this problem.